Machines using fluid motors (e.g. pistons and cylinder assemblies) for clamping, actuating tools or welding guns, etc. are generally either pneumatically or hydraulically powered. If high speed operation of the fluid motor is required, compressed air is the preferred power source because it is cleaner and gives faster action than pressurized oil at the same supply pressure. The source for compressed air is normally a facility wide supply system and the compressed air is provided at a system pressure. This system pressure may or may not be compatible with the amount of work required by the fluid motors. Where a large amount of energy is required, any attempt to utilize the system pressure will result in air cylinder sizes which are too large for many applications. Pressurized oil can be used at higher pressures than compressed air, thereby reducing cylinder sizes, but to get fast action, relatively large displacement pumps are required. Hydraulic power units for these systems tend to be large and costly, and because they have continuously operating pumps, they consume substantial amounts of power, are a source of continuous noise and generate considerable heat because all excess oil is pumped back to the reservoir after being raised to working pressure. This continuous operation of the hydraulic power units occurs even during the dwell or rest portion of the load cycle, because the pumps must be able to quickly meet any sudden demand. Variable displacement pumps may consume only a moderate amount of power, but these pumps are even more costly to purchase initially.
It is therefore an object of the present invention to provide an improved fluid power supply which operates on demand only basis to combine the load being exerted by a primary cylinder with the load exerted by a secondary cylinder.
It is a further object of the present invention to provide a third cylinder coupled to the primary and secondary cylinders to provide pressurized fluid at a pressure which is greater than the supply pressure on a demand only basis.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.